The invention disclosed herein generally relates to bond coatings and thermal barrier coatings applied to metals. The metals are frequently portions of components used in turbine engines. The invention also relates to processes for depositing such coatings.
Components formed of specialty materials like superalloys are used in various industrial applications, under a diverse set of operating conditions. In many cases, the components are provided with coatings which impart several characteristics, such as corrosion resistance, heat resistance, oxidation resistance, and wear resistance. As an example, the various components of turbine engines, which typically can withstand in-service temperatures in the range of about 1100° C.–1150° C., are often coated with thermal barrier coatings (TBC's), to effectively increase the temperature at which they can operate.
Most TBC's are ceramic-based, e.g., based on a material like zirconia (zirconium oxide), which is usually chemically stabilized with another material such as yttria. For a jet engine, the coatings are applied to various superalloy surfaces, such as turbine blades and vanes, combustor liners, and combustor nozzles. Usually, the TBC ceramics are applied to an intervening bond coating (sometimes referred to as a “bond layer” or “bond coat”) which has been applied directly to the surface of the metal part. The bond coat is often critical for improving the adhesion between the metal substrate and the TBC.
The effectiveness of a TBC is often measured by the number of thermal cycles it can withstand before it delaminates from the substrate which it is protecting. In general, coating effectiveness decreases as the exposure temperature is increased. The failure of a TBC is often attributed to weaknesses or defects related in some way to the bond coating, e.g., the microstructure of the bond coating. TBC failure can also result from deficiencies at the bond coating-substrate interface or the bond coating-TBC interface.
The microstructure of the bond coating is often determined by its method of deposition. The deposition technique is in turn determined in part by the requirements for the overlying protective coating. For example, many TBC's usually require a very rough bond coat surface (e.g., a root mean square roughness (Ra) value of greater than about 200 micro-inches), for effective adhesion to the substrate. An air plasma spray (APS) technique is often used to provide such a surface.
There continues to be a need in the art for bond coatings which provide very good adhesion between the substrate and a subsequently-applied TBC, e.g., bond coatings with a relatively rough surface. Furthermore, new processes for applying and curing such coatings in regions of a substrate which are somewhat inaccessible are also of great interest. (Conventional thermal spray equipment is sometimes too large and cumbersome for such regions). Moreover, the entire TBC system—bond coating with the TBC itself—should exhibit good integrity during exposure to high temperatures and frequent thermal cycles. Such a system should be effective in protecting components used in high performance applications, e.g., superalloy parts exposed to high temperatures and frequent thermal cycles.